The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.
X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Conventional X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a position-sensitive detector, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).
A method for analyzing the X-ray data to determine film thickness is described, for example, in U.S. Pat. No. 5,740,226, to Komiya et al., whose disclosure is incorporated herein by reference. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
Various types of position-sensitive X-ray detectors are known in the art of reflectometry. Solid-state arrays typically comprise multiple detector elements, which are read out by a CCD or other scanning mechanism. Typically, each element accumulates photoelectric charge over a period of time before being read out and therefore cannot resolve the energy or number of incident X-ray photons. XRR systems known in the art that are based on such arrays simply record the total integrated radiation flux that is incident on each element. The signals at low angles, below the total external reflection angle, are usually much stronger than the signals above this angle. A ratio of 105 to 107 in photon flux between 0xc2x0 and 3xc2x0 reflections is typical. The dynamic range of array detection systems known in the art is substantially smaller than this ratio. Consequently, high-order fringes at higher incidence angles cannot generally be detected. Photon counting sensitivity is needed in order to measure the weak signals at these angles.
A further drawback of X-ray thin film measurement systems known in the art is their lack of spatial resolution. X-ray optics, such as the above-mentioned curved monochromators, are capable of focusing an X-ray beam to a spot diameter below 100 xcexcm. When the beam is incident on a surface at a low angle, below 1xc2x0, for example, the spot on the surface is elongated by more than 50 times this diameter. A measurement that is made under these circumstances provides only an average of surface properties over the entire elongated area. For many applications, such as evaluating thin film microstructures on integrated circuit wafers, better spatial resolution is required.
Although the present patent application is concerned mainly with systems in which a sample is irradiated by a monochromatic beam, other methods for X-ray reflectometry are also known in the art. One such method is described, for example, in an article by Chihab et al., entitled xe2x80x9cNew Apparatus for Grazing X-ray Reflectometry in the Angle-Resolved Dispersive Mode,xe2x80x9d in Journal of Applied Crystallography 22 (1989), p. 460, which is incorporated herein by reference. A narrow beam of X-rays is directed toward the surface of a sample at grazing incidence, and a detector opposite the X-ray beam source collects reflected X-rays. A knife edge is placed close to the sample surface in order to cut off the primary X-ray beam, so that only reflected X-rays reach the detector. A monochromator between the sample and the detector (rather than between the source and sample, as in U.S. Pat. No. 5,619,548) selects the wavelength of the reflected X-ray beam that is to reach the detector.
It is an object of the present invention to provide improved methods and systems for X-ray analytical measurements, and particularly for measurements of thin film properties.
It is a further object of some aspects of the present invention to provide systems for X-ray reflectometry with enhanced dynamic range.
It is still a further object of some aspects of the present invention to provide systems for X-ray microanalysis with enhanced spatial resolution.
In preferred embodiments of the present invention, a system for X-ray reflectometry is used to determine properties of thin films on the surface of a sample, typically a semiconductor wafer. The sample is irradiated by a monochromatic beam of X-rays, which is focused to a small spot size on the surface of the sample. X-rays reflected from the surface are incident on a detector array, preferably a CCD array, each detector element in the array corresponding to a different angle of reflection from the surface. Charge stored by the detector elements is clocked out of the array to a processor, which analyzes the charges to derive a fringe pattern, corresponding to the intensity of X-ray reflection from the surface as a function of angle. The X-ray optics and processing circuitry in the system are arranged to achieve a high dynamic range, whereby high-order fringes are plainly apparent in the reflected signal. The processor analyzes the fringe pattern based on a physical model of thin film properties, including density, thickness and surface roughness. The high dynamic range enables the system to determine these properties accurately not only for the upper thin film layer, but also for one or more underlying layers on the surface of the sample.
As noted in the Background of the Invention, one of the major problems in X-ray reflectometry systems is that surface reflectivity is much greater at low angles (near 0xc2x0) than at higher angles. The strong flux of low-angle X-rays that is incident on the detector array tends to increase the overall background level, making it difficult to detect the weaker high-angle flux. To address this problem, in some preferred embodiments of the present invention, the system comprises a dynamic shutter, having low-angle and high-angle detection positions. In the low-angle position, the shutter allows the entire beam of X-rays from the X-ray source to impinge on the sample. In the high-angle position, however, the shutter blocks low-angle X-rays. As a result, the intense low-angle reflections are eliminated, so that the background level at the detector is reduced, and useful signals can be derived at high angles. The processor xe2x80x9cstitches togetherxe2x80x9d the signals received by the detector in the low- and high-angle positions of the shutter in order to derive a single fringe spectrum with enhanced dynamic range.
Preferably, the housing and readout circuits associated with the detector array are also designed to reduce the background level in the high-angle elements. Most preferably, the CCD array is coupled to the readout circuits so that the signal output from the array to the circuits is adjacent to the end of the array that receives the high-angle reflections. In this configuration, the high-angle array elements are read out first, giving accurate readings of the weak signals received by these elements before the readings are contaminated by background charge transferred from the low-angle elements. Additionally or alternatively, the detector array is protected by an evacuated enclosure, closed off by an X-ray transparent window spaced a substantial distance in front of the array, between the array and the sample. As a result, scattering of low-angle X-rays from the window and from the air in the space immediately in front of the array is substantially reduced, thus further reducing the background level at the high-angle elements.
As a further means for reducing both the size of the incident X-ray spot on the surface of the sample and the excessive signal at low incidence angles, in some preferred embodiments of the present invention, a dynamic knife edge is positioned over the surface. Preferably, the dynamic knife edge is operated in conjunction with the above-mentioned dynamic shutter. For measurements at low incidence angles, the knife edge is lowered very near to the surface, intercepting the incident X-ray beam and thus shortening the lateral dimension of the spot on the surface (i.e., the dimension in the direction along the surface that is roughly parallel to the beam axis). For high-angle measurements, at which the dynamic shutter is used, the knife edge is preferably raised out of the way, to allow the full intensity of the X-ray beam to be used. Such operation of the knife edge allows measurements to be made with high spatial resolution, particularly at low angles at which the lateral dimension of the spot is most greatly elongated, while maintaining high sensitivity even at high angles.
In some preferred embodiments of the present invention, the processor analyzes the detector array output so as to determine a total, lumped X-ray flux for each detector element at high intensities (typically at low angles), while effectively counting individual X-ray photons per element at low intensities (high angles). The inventors have found that an X-ray photon of a known energy will generate a certain average charge in a detector element when the photon is incident as part of a high photon flux, and a lower average charge, which may be spread over two adjacent detector elements, at low flux. Preferably, to analyze the array output, the processor first determines the number of photons incident on each detector element that encountered a high X-ray flux, by dividing the total charge accumulated in these elements by the high-flux average charge. Then, over the remaining detector elements, the processor seeks individual elements or pairs of elements having charge levels comparable to the low-flux charge. The processor records a single photon count for each such element or pair of elements. This technique allows a single detector array, such as a CCD array, to be used simultaneously for both lumped flux and photon counting measurements, thus further enhancing the dynamic range with which the system can measure the pattern of reflection fringes.
Although preferred embodiments of the present invention are directly mainly toward enhancing X-ray reflectometric measurements on thin films, and particularly on semiconductor wafers, the principles of the present invention can similarly be used in other applications of X-ray reflectometry, as well as in other types of radiation-based analysis.
There is therefore provided, in accordance with a preferred embodiment of the present invention, reflectometry apparatus, including:
a radiation source, adapted to irradiate a sample with radiation over a range of angles relative to a surface of the sample;
a detector assembly, positioned to receive the radiation reflected from the sample over the range of angles and to generate a signal responsive thereto; and
a shutter, adjustably positionable to intercept the radiation, the shutter having a blocking position, in which it blocks the radiation in a lower portion of the range of angles, thereby allowing the reflected radiation to reach the array substantially only in a higher portion of the range, and a clear position, in which the radiation in the lower portion of the range reaches the array substantially without blockage.
Preferably, the radiation includes X-ray radiation, and the lower portion of the range includes angles below a critical angle for total external reflection of the radiation from the surface.
Further preferably, the reflected radiation is characterized by a variation of intensity as a function of the angle due to the thin film layers, and when the shutter is in the blocking position, the signal generated by the detector assembly responsive to the reflected radiation in the higher portion of the range of angles has a reduced background level relative to the background level when the shutter is in the clear position. In a preferred embodiment, the sample includes one or more thin film layers, and the variation of intensity includes an oscillatory pattern. Most preferably, the apparatus includes a processor, which is coupled to receive the signal from the detector assembly and to analyze the oscillatory pattern to determine one or more properties of the one or more thin film layers.
Additionally or alternatively, the oscillatory pattern includes an initial shoulder occurring near a critical angle for total external reflection of the radiation from an outer one of the thin film layers at the surface of the sample, and the one or more properties includes a density of the outer thin film layer, such that the processor is adapted to estimate the density of the outer thin film layer responsive to the shoulder, irrespective of any other one of the properties.
Further additionally or alternatively, the detector assembly includes an array of detector elements, and the signal is indicative of respective charges accumulated by the detector elements due to photons of the radiation that are incident on the elements, and the processor is adapted to estimate, responsive to the respective charges, a number of the photons that was incident on each of the elements.
Preferably, the detector assembly is adapted to receive the radiation over a first integration period while the shutter is in the clear position and over a second integration period, substantially longer than the first integration period, while the shutter is in the blocking position, and including a processor, which is coupled to receive the signal from the detector assembly and to combine the signal generated by the detector assembly during the first integration period with the signal generated by the detector assembly during the second integration period so as to reconstruct the variation of the intensity over the entire range of angles.
Preferably, the detector assembly includes an array of detector elements, including a first element positioned to receive the radiation reflected from the sample in the lower portion of the range of angles and a last element positioned to receive the radiation reflected from the sample in the higher portion of the range of angles. Most preferably, the detector assembly includes a readout circuit and a charge coupled device (CCD), which has an output connected to the readout circuit and is coupled to transfer charges generated by the detector elements responsive to the radiation from the detector elements to the output in sequence along the array beginning with the last element.
Preferably, the radiation source is adapted to irradiate a spot on the sample, and the apparatus includes a knife edge, adjustably positionable to block a portion of the radiation while the shutter is in the clear position, so as to reduce a dimension of the spot. Most preferably, the radiation includes X-ray radiation, and the range includes angles in a vicinity of a critical angle for total external reflection of the radiation from the surface, and the knife edge is positionable so as to reduce the dimension of the spot to no more than 1 mm.
There is also provided, in accordance with a preferred embodiment of the present invention, radiation sensing apparatus, including:
a detector assembly, including an array of detector elements, positioned to receive X-ray photons emitted over a range of angles and to generate a signal indicative of respective charges accumulated by the detector elements due to the photons that are incident on the elements; and
a processor, which is coupled to receive the signal from the detector assembly and to determine, responsive to the signal, whether a high flux of the photons or a low flux of the photons was incident on each of the elements, and to estimate the number of photons incident on each of the elements by dividing the charges accumulated by the elements on which the high flux was incident by a high-flux average charge, and dividing the charges accumulated by the elements on which the low flux was incident by a low-flux average charge, substantially different from the high-flux average charge.
Preferably, the low flux is considered to be incident on one of the elements when no more than a single one of the photons is incident on the element over a period during which the charges are accumulated. Most preferably, for the elements on which the low flux was incident, the processor is adapted to divide the charges accumulated by a mutually-adjacent pair of the elements by the low-flux average charge, so as to determine whether one of the photons was incident on one of the elements in the pair.
Additionally or alternatively, the detector assembly is adapted to receive the X-ray photons reflected by a sample over the range of angles, characterized by a variation of flux of the reflected photons as a function of angle, such that the high flux is incident on the elements in a low-angle portion of the range, and the low flux is incident on the elements in a high-angle portion of the range.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a detector assembly, including:
an array of detector elements, positioned to receive radiation and to generate a signal responsive thereto, the array including a first element and a last element and having a length defined by a distance between the first and last elements; and
an evacuable enclosure having a front side and a rear side separated by a distance that is at least equal to the length of the array, wherein the array is positioned at the rear side of the enclosure, and the enclosure includes a window at a front side thereof, which is adapted to allow the radiation to pass therethrough so as to impinge on the array.
Preferably, the front and rear sides of the enclosure are separated by a distance of at least twice the length of the array.
Further preferably, the radiation is emitted from a sample outside the enclosure, wherein the radiation includes X-rays reflected from the sample over a range of angles, such that the first element receives the radiation reflected from the sample in a lower portion of the range of angles and the last element receives the radiation reflected from the sample in a higher portion of the range of angles. Most preferably, the detector assembly includes a readout circuit and a charge coupled device (CCD), which has an output connected to the readout circuit and is coupled to transfer charges generated by the detector elements responsive to the radiation from the detector elements to the output in sequence along the array beginning with the last element.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for reflectometry, including:
irradiating a sample with radiation over a range of angles relative to a surface of the sample;
receiving the radiation reflected from the sample over the range of angles so as to generate a low-range signal responsive to the radiation reflected in a lower portion of the range;
blocking a lower part of the range of angles, thereby allowing the reflected radiation to reach the array substantially only in a higher portion of the range;
receiving the radiation reflected from the sample over the range of angles while the lower portion of the range is blocked, so as to generate a high-range signal responsive to the radiation reflected in a higher portion of the range; and
combining the high-range and low-range signals to determine a pattern of the reflected radiation over the range of angles, including both the lower and higher portions.
There is moreover provided, in accordance with a preferred embodiment of the present invention, a method for sensing radiation, including:
receiving X-ray photons emitted over a range of angles at an array of detector elements, so as to generate a signal indicative of respective charges accumulated by the detector elements due to the photons that are incident on the elements; and
determining, responsive to the signal, whether a high flux of the photons or a low flux of the photons was incident on each of the elements;
estimating the number of photons incident on each of the elements on which the high flux was incident by dividing the charges accumulated by the elements by a high-flux average charge; and
estimating the number of photons incident on each of the elements on which the low flux was incident by dividing the charges accumulated by the elements by a low-flux average charge, substantially different from the high-flux average charge.
Preferably, determining whether the high flux or the low flux was incident includes determining that the high flux was incident on one of the elements when the charge accumulated by the element, not including a background charge, is at least three times the high-flux average charge. Additionally or alternatively, determining whether the high flux or the low flux was incident includes determining that the low flux was incident on one of the elements when no more than a single one of the photons was incident on the element over a period during which the charges were accumulated.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, a method for detecting radiation, including:
enclosing an array of detector elements in an enclosure, the array including a first element and a last element defining a length of the array therebetween, the enclosure having a window at a front side thereof, which is adapted to allow radiation to pass therethrough, and which is positioned at a distance from the array that is at least equal to the length of the array;
evacuating the enclosure containing the array; and
receiving the radiation at the array and generating a signal responsive thereto.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for reflectometry, including:
irradiating a sample including one or more thin film layers with radiation over a range of angles relative to a surface of the sample;
receiving the radiation reflected from the sample over the range of angles and generating a signal responsive to the reflected radiation, the signal having an oscillatory pattern as a function of the angle, the pattern including an initial shoulder occurring near a critical angle for total external reflection of the radiation from an outer one of the thin film layers at the surface of the sample; and
estimating a density of the outer thin film layer responsive to the shoulder, irrespective of any other properties of the one or more thin film layers.
Preferably, the method includes determining one or more of the other properties using the estimated density, wherein determining the one or more of the other properties includes estimating a thickness of at least one of the layers and/or a surface roughness of at least one of the layers.